The present invention relates to the selective elimination of trans-unsaturated double bonds in fatty acid compounds from a substrate containing cis- and trans-isomers of said fatty acid compounds. The elimination takes place by adsorption of said fatty acid compounds by zeolite materials having a selectivity towards trans-isomers. The invention also relates to the elimination of trans-isomers of said compounds by their selective adsorption by said zeolite materials further containing a metal catalyst, and saturation of the double bond of the fatty acid compounds adsorbed in the pores or cavities of said zeolite.
The aforementioned trans-unsaturated fatty acid compounds may be saponifiable or non saponifiable molecules. The saponifiable fatty acid compounds comprise esters, mono-, di- and triglycerides, phospholipids, glycolipids, diol esters of fatty acids, waxes and sterol esters. The non saponifiable compounds comprise free fatty acids, sterols, carotenoids, monoterpenes and tocopherols. Other fatty acid compounds well known for their amphiphilic properties are fatty acid derivatives like fatty alcohols, fatty amines or fatty acid dimers. The present invention relates, in particular, to the elimination of trans-unsaturated fatty acid residues in triglycerides from edible oils and fats and is therefore mostly related to food technology.
Oils and fats for food applications are mainly triglycerides: molecules having three fatty acids esterified with glycerol. In most cases, the fatty acid chains are not branched, have a chain length of 4 to 24 carbon atoms, and may contain up to three double bonds. The physicochemical properties of triglycerides strongly depend on the chemical structure of the fatty acid residues and more particularly on their chain length and the amount of double bonds present. In fact, the melting point of triglycerides increases with increasing chain length and decreasing unsaturation of the fatty acid residues present.
Hardening by hydrogenation is a common process to increase the melting profile of edible oils and fats. In most cases, this process is carried out as a heterogeneous reaction with hydrogen gas and a heterogeneous catalyst. Often used catalyst materials are metals like nickel or palladium finely deposited on carriers like kieselguhr or silica. The hydrogenation process is typically carried out in agitated batch autoclaves at temperatures above 400 K and hydrogen gas pressures above 0.1 MPa. Agitation can be realised by stirring or by more complex systems like circulating the reactors content through a venturi system where hydrogen is mixed intensively with the oil being hydrogenated. Continuous processes are used as well.
Hydrogenation can be carried out to accomplish complete saturation of all double bonds present. In many cases, however, partial hydrogenation is aimed for. In the latter case and with the processes actually used industrially, isomerisation of carbon-carbon double bonds in the fatty acid residues occurs besides the saturation of double bonds by the addition of hydrogen. For food applications, starting materials for hydrogenation are of biological origin. In such materials, like palm oil, soybean oil, softseed oils and the like, almost no trans-isomers are present; the position of the double bonds in the fatty acid chains is well defined too. Hydrogenation by means of metal catalysts like nickel, palladium, platinum, ruthenium, rhodium and others, inevitably leads to cis/trans-isomerisation, since the reaction mechanism using such catalysts implies a transition state with a freely rotating semi-hydrogenated configuration (L. F. Albright, J. Am. Oil Chem. Soc., 40/5, 16 (1963) and G. Cecchi, G. Mallet, E. Ucciani, Riv. Ital. Sost. Grasse, 58/5, 228 (1981) and R. R. Allen, J. Am. Oil Chem. Soc., 63/10, 1328 (1986)). The existence of this transition state also leads to positional isomerisation of double bonds when the total addition of hydrogen is sufficiently slow. Just like saturated fatty acids, such isomeric fatty acid residues increase also to some extent the melting profile of triglycerides.
Although hydrogenation is the main cause for the presence of fatty acid isomers in food oils and fats, similar isomers can be found in other lipids too. Animal fats like butter fat or tallow have some trans-isomers, and fully refined, non-hydrogenated fats also may contain a very low content of said isomers due to the high temperature processing on refining and deodorisation (L. H. Wesdorp, Lipid Techn., 8/6, 129 (1996)). However, the amount of isomers present in all these is significantly lower than one can expect in general in hardened products.
According to recent studies, a lot of controversy has been risen about possible health hazards of these trans-unsaturated fatty acids (M. B. Katan, P. L. Zock, R. P. Mensink, Annu. Rev. Nutr., 15, 473, (1995) and British Nutrition Foundation, "Trans Fatty Acids", (1995)).
For that reason, attempts have been made to reduce the trans fatty acid content in food products. An important contribution is the reduction of these trans fatty acids in hydrogenated edible fats (J. M. Hasman, Inform, 6/11, 1206 (1995)). A lot of hydrogenation process modifications have been proposed to achieve this objective.
The hydrogenation process of vegetable oils is generally carried out in an agitated batch autoclave. Typical process parameters are a hydrogen pressure ranging from 0.1 to 0.5 MPa and a hydrogenation temperature ranging from 400 to 475 K. Since isomerisation depends on the concentration and lifetime of the so-called semi-hydrogenated transition state, a first approach to reduce isomerisation relies on increasing the hydrogen concentration on the catalyst active sites. This can typically be realised by a higher pressure of hydrogen gas supplied, to increase its solubility in the oil, and by increasing the hydrogen mass transfer coefficient, by more efficient agitation (P. R. Puri, J. Am. Oil Chem. Soc., 55/12, 865 (1978) and J. W. E. Coenen, Riv. Ital Sost. Grasse, 58/9, 445 (1981)). Similarly, a reduction of the reaction temperature has been proven to have some effect on the isomerisation of double bonds, more specifically a suppression of trans double bond formation, but also brings along a reduced reaction velocity. Both means, increase of hydrogen concentration and temperature lowering, although effective to some extent in lowering the concentration of isomerised products in hardened oils and fats, can not eliminate isomers, mainly transisomers, in said oils and fats.
A second approach one has followed to reduce trans-unsaturated fatty acid compounds is to influence the catalytic system itself The metal particles commonly used are as small as 20-50 .ANG. in order to provide a high reaction surface area. To modify the catalytic properties of the metallic catalyst, alloying (P. N. Rylander, J. Am. Oil Chem. Soc., 47, 482 (1970) and A. I. Thomson, J. Chem. Tech. Biotech., 37, 257 (1987) and J. D. Parry, J. Chem. Tech. Biotech, 50, 81 (1991)) or addition of modifiers like amine or ammonium compounds (U.S. Pat. No. 4,307,026 and U.S. Pat. No. 4,228,088 and EP-A-0,576,477 and E. Draguez de Hault, J. Am. Oil Chem. Soc., 65, 195, (1984)) have been used to decrease the isomerisation effects like trans double bond formation.
Still other means have been used to influence the concentration of trans-unsaturated fatty acid compounds. Homogeneous catalysis with metal complexes like benzoate-Cr(CO).sub.3 or triphenylphosphine complexes of ruthenium or rhodium has been investigated on small scale (E. N. Frankel, J. Am. Chem. Soc., 90, 2446, (1968) and C. Bello, Ibid., 62, 1587, (1985) and E. A. Emken, J. Am. Oil Chem. Soc., 65, 373, (1988)). A reduction of the trans fatty acid formation upon hydrogenation was realised. Industrial use, however, can not be expected since the catalysts used, could be toxic and could not be removed easily and economically after hydrogenation. Attempts to heterogenise the catalytic systems mentioned were not successful
Besides changing of the hydrogenation process parameters, use of modifiers, alteration of the catalytic metal function and specific supporting of the metal to decrease the trans-isomeric fatty acid content upon hydrogenation, has been studied. Supporting the metal on different materials like titaniumdioxide and kieselguhr has been published (E. Draguez de Hault, J. Am. Oil Chem. Soc., 65, 195 (1984). A high dispersion of the metal in a porous structure has been accomplished according to EP-A-0,233,642, U.S. Pat. No. 4,584,139 and U.S. Pat. No. 5,492,877. However, none of these could eliminate trans-unsaturated fatty compounds from the substrates studied completely.
Still further attempts have been carried out to influence the cis/trans-isomerisation during hydrogenation. Electrocatalytic hydrogenation by the addition of hydrogen donors, although decreasing isomerisation, still produced trans-isomers; in addition, the velocity of the reaction was lower (EP-A-0,429,995 and U.S. Pat. No. 4,399,007 and G. J. Yusem, J. Am. Oil Chem. Soc.).
Hydrogenation of fats and oils on zeolites has been reported by Koritala (S. Koritala, J. Am. Oil Chem. Soc., 45, 197, (1968)). Pt/Na--Y zeolite was found to be able to hydrogenate triglyceride samples; cis/trans-isomerisation, however, could not be eliminated (A. Brehm and H. M. Polka, Chem.-Ing. Tech., 61, 963, (1989)). Pd/CuO/ZnO/ZSM-5 has been used to hydrogenate methyllinoleate, although with low conversion, without any trans-isomer formation; this catalyst wasn't very active for the hydrogenation of triglycerides (R. Muller, "Selektieve Hydrierung von Olen an suspendierten Katalysatoren", MSc thesis, Oldenburg, (1991)). The authors report on the introduction of a hydrogenation selectivity for a palladium loaded zeolite material by having copperoxide and zincoxide inside the pores as well. During the preparation of said metal loaded zeolite material, after the oxidation step, a subsequent reduction below 473 K is required to have a catalytically active palladium while the oxides of copper and zinc, which are required to obtain the selectivity, survive this reductive operation. The elimination of trans isomers with this metal alloy loaded catalyst, however, can not be attributed to selective adsorption of said trans fatty acid isomers, but by excluding formation of trans-unsaturated fatty acid compounds. The effect, also according to the authors, must be the result of the restricted mobility of the semi-hydrogenated transition state when present in the pores of the zeolite material. To increase activity, they recommend to widen the pores of the zeolites. This, however, will certainly not increase the selectivity for the adsorption of trans fatty acid compounds.
It should be clear that the above mentioned methods to avoid isomerisation, mainly cis/trans-isomerisation, are just partial solutions for what one should aim for: a substrate without any residual trans-unsaturated fatty acid compounds left. The present invention will overcome all drawbacks of the prior art in providing a process for the adsorption of transunsaturated fatty acid compounds from substrates containing cis- and trans-isomers of said compounds by a microporous zeolite material having a selectivity for the adsorption of said trans-unsaturated compounds bigger than one.